WASHINGTON (CNN) -- President Bush will announce Tuesday a plan to withdraw about 8,000 U.S. troops from Iraq over the next several months even as he beefs up the U.S. military presence in Afghanistan.

In a transcript of remarks given Monday to the news media and slated for delivery Tuesday evening to the National Defense University in Washington, Bush says he is making the decision based on a recommendation from top military officers, including Gen. David Petraeus, the highest-ranking U.S. military officer in Baghdad.

"He and the Joint Chiefs of Staff have recommended that we move forward with additional force reductions," the transcript says.

"Over the next several months, we will bring home about 3,400 combat support forces -- including aviation personnel, explosive ordnance teams, combat and construction engineers, military police, and logistical support forces. By November, we will bring home a Marine battalion that is now serving in Anbar province. And in February of 2009, another Army combat brigade will come home.

"This amounts to about 8,000 additional American troops returning home without replacement. And if the progress in Iraq continues to hold, Gen. Petraeus and our military leaders believe additional reductions will be possible in the first half of 2009."

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Bush cites what he says has been a reduction of violence in Iraq to its lowest point since spring 2004. "Civilian deaths are down, sectarian killings are down, suicide bombings are down and normal life is returning to communities across the country," he says.

Though progress "is still fragile and reversible," he says, Petraeus and U.S. Ambassador to Iraq Ryan Crocker "report that there now appears to be a 'degree of durability' to the gains we have made."

"While the enemy in Iraq is still dangerous, we have seized the offensive, and Iraqi forces are becoming increasingly capable of leading and winning the fight," he adds. "As a result, we have been able to carry out a policy of 'return on success' -- reducing American combat forces in Iraq as conditions on the ground continue to improve."

But Bush says more work needs to be done in Afghanistan, where the number of U.S. forces has risen from 20,000 two years ago to 31,000 today.

In November, a Marine battalion that had been poised to be sent to Iraq will instead be sent to Afghanistan and an Army combat brigade of several thousand fighters will go to Afghanistan in January, he says.

"They will help clarify a stark contrast in Afghanistan: While the terrorists and extremists deliberately target and murder the innocent, coalition and Afghan forces risk their lives to protect the innocent," he says.

But the president acknowledges the U.S. military is sometimes responsible for civilian deaths, which Afghanistan President Hamid Karzai has complained bitterly about.

"Regrettably, there will be times when our pursuit of the enemy will result in accidental civilian deaths," Bush says. "I have given President Karzai my word that America will work closely with the Afghan government to ensure the security of the Afghan people while protecting innocent life."